A. Field of the Invention
The present invention relates to a method for submerged-arc welding a very low carbon steel for obtaining line pipe with high strength, high toughness and high performance suitable for low temperature applications.
B. Description of the Prior Art
In the development of line pipe steel suitable for use in cold climate, remarkable progress is being made in lowering the C eq. of low alloy high strength steels, and particularly in lowering the carbon content of such steels to a very low percentage so as to obtain steels with good weldability and low temperature toughness.
Further, advances in controlled rolling technology have made available various low-carbon steels containing not more than 0.06%C(as compared with ordinary steels which contain as much as 0.1%C).
At the present time, line pipe is being produced from high strength low alloy steels, using the submerged-arc welding process, wherein with a view to improving low temperature toughness, a welding flux to which Ti and B are added and a welding wire of one of the following specified compositions are used in combination. The compositions of the welding wires are as follows:
TABLE 1 ______________________________________ (wt %) C Si Mn Mo ______________________________________ JIS SAW32 .ltoreq.0.15 0.05-0.25 1.30-1.80 # JIS SAW41 .ltoreq.0.17 .ltoreq.0.05 1.80-2.20 # AWS Standard EA3 0.10-0.18 .ltoreq.0.05 1.70-2.40 0.45-0.65 AWS Standard EH14 0.10-0.18 0.05-0.30 1.75-2.25 -- ______________________________________ Note: JIS: Japanese Industrial Standard SAW: Submerged Arc Welding AWS: American Welding Society #: Mo and/or Ni, etc. included in addition to JISspecified components.
In making a tubular product of the above steel, various processes, such as the UOE (U-ing and O-ing expander), spiral seam pipe making, and the bending roll method are being used.
In accordance with the conventional method, a low alloy steel containing a low carbon content of 0.05% was subjected to controlled rolling to produce a steel sheet which was then formed into a tubular shape. The tubular product was subjected to submerged-arc welding with the combined application of a welding wire containing 0.10%C, 2.0%Mn and 0.5%Mo corresponding to an AWS standard EA3 wire, and a melt type flux of the TiO.sub.2 and B.sub.2 O.sub.3 containing system. Mechanical properties of the weld metal thus obtained were examined. The results obtained are shown in Table 2.
TABLE 2 ______________________________________ Results of 2 mm V Combinations Test Notch Charpy Steel Sheet Welding Wire Welding Flux Impact Test ______________________________________ Very low 0.1% C-2% Melt flux of Number of Tests 60 C steel* Mn-0.5% Mo TiO.sub.2 --B.sub.2 O.sub.3 Average value 18.1 kg-m Fluctuation .sigma. = 3.5 Ordinary C 0.1% C-2% Melt flux of Number of Tests 60 steel** Mn-0.5% Mo TiO.sub.2 --B.sub.2 O.sub.3 Average value 20.5 kg-m Fluctuation .sigma. = 0.9 ______________________________________ *Steel of 0.05 C0.3 Si1.5 Mn0.2 Ni0.25 Mo0.05 Nb0.07 V0.03 Al0.01 Ti **Steel of 0.09 C0.3 Si1.6 Mn0.3 Cu0.3 Ni0.05 Nb0.09 V0.03 Al
The investigations carried out by the inventors revealed a reduction in the high temperature ductility of the weld metal and a tendency toward promotion of the occurrence of defects in the weld metal. Consequently, as indicated by Table 2 the fluctuation in the low temperature toughness becomes greater than that of the weld metal of an ordinary steel containing the usual amount of carbon.
On the other hand, the ordinary low alloy steel containing 0.09%C welded by the above-mentioned submerged-arc welding process undergoes no reduction in high temperature toughness at all. And, as a result, the fluctuation in its low temperature toughness is small, as also shown in Table 2. In this way it was demonstrated that the problem inherent in the weld metal in welding very low C steel results from the combination of a welding wire containing an ordinary amount of C and a very low carbon steel.